A randomized, open-label equivalence study of FTC versus lamivudine in patients on a stable triple antiretroviral therapy regimen containing lamivudine, stavudine or zidovudine, and a protease inhibitor or non-nucleoside reverse transcriptase inhibitor (Triangle Pharmaceutical Protocol FTC-303). This study is a phase III, open-label, randomized, parallel group, multi-center study of FTC vs. lamivudine in HIV-1 infected patients with plasma HIV-1 RNA <400 copies/mL on a stable antiretroviral therapy regimen containing lamivudine, stavudine or zidovudine, and a PI or NNRTI. Patients are considered stable if they have maintained a triple therapy for > 12 weeks. Four hundred fifty patients who meet the study entry criteria will be randomized in a 2:1 ratio to one of two treatment arms: ARM 1 (n-300) - replace lamivudine with (200 mg qd) while continuing with the same background regimen; and ARM 2 (n=150) - continue on the same lamivudine (150 mg bid) containing regimen. In addition, patients will be stratified based upon HIV-1 RNA levels determined at the Screening Visit using the Roche Ultrasensitive assay as follows: HIV-1 RNA levels <50 copies/mL and > 50 copies/mL. Approximately 45 sites will be used and all patients will be treated on an out-patient basis with the exception of an optional 12-hour inpatient pharmacokinetic substudy. Once patients have been randomized, they will receive study drug over a 48-week period returning for clinic visits at weeks 2, 4, 8, 12, 16, 20, 24, 28, 32, 40, 44, 48. At each visit, plasma samples will be drawn for determination of viral load. Data from the first 100 patients randomized will be used for an interim analysis. The analysis will be made after all patients in the interim group have either completed 24 weeks of treatment or withdrawn from the study. A patient will complete the study after 48 weeks on therapy. All patients who complete the study maintaining virologic success through Week-48 will be offered access to FTC.